Regeneration of Reactive Pd Surfaces in Au-Pd Nanoparticles after Electrochemical Aging

One major hurdle in the development of efficient electrocatalysts is deterioration of activity, which can be caused by selective dissolution of metals [1]. The recent development of gold-palladium (Au-Pd) alloy nanoparticles outlines a contemporary strategy to impart increased stability in nanoparticles by combining a more electrochemically stable metal such as Au with lesser stable Pd [2, 3]. Unfortunately, the relative surface-reactivity of Au is poor, and as a result, there is an inevitable design constraint to form Pd-rich outer shell structure - a task that is particularly daunting considering the high dissolution tendency of Pd [4]. Here we present an electrochemical aging method that allows for increasing the Pd concentration at the surface of Au-Pd NPs by means of diffusion of Pd atoms from the particle-core to the shell. The method involves an electrochemical aging treatment in alkaline KOH media for which the choice of suitable upper vertex potential proved to be particularly very important. Au-Pd NPs synthesized by means of liquid-phase laser-ablation method [5] were used in this study, typically ranging in diameter between 14.5 nm to 22.6 nm and composition between Au/Pd: 20/80 to 80/20 ratios. The particle diameter remained homogeneous across the whole composition range, thanks to the size-selectivity of laser-ablation synthesis of alloy molar fraction series, in line with the literature [5]. This allows for excluding any size-effects on the electrochemical behaviour of alloy nanoparticles. The nominal Au/Pd compositions initially fed to the laser appeared to be well preserved down to single-nanoparticle level as confirmed by detailed analyses of the SEM/EDX and XPS spectroscopic data. Formation of Au/Pd solid-solution was also apparent from the estimation of the lattice-parameters using X-ray diffraction (XRD) that varied linearly in accordance with the change in Pd alloy content. The cyclic voltammograms recorded in 0.1M N2-saturated KOH revealed features characteristic of both Au and Pd surfaces, e.g., the reduction peaks at 0.98 V and 0.55 V vs RHE (respectively), which was reflective of a mixed Au and Pd surface-structure exposed to the electrochemical treatments. Importantly, both the peak-positions and the integrated surface-charge shifted in correspondence with the variable Pd content in the alloys, thus serving as the initial markers of the surface-state before further electrochemical treatments. Marked differences could be noted between the electrochemical behaviours in acid and the alkaline media, as well as the potential range and the number of cycles involved ( Figure 1 ). The initial H-desorption/adsorption peaks (A, B) and the PdO reduction peaks apparent at the onset of aging in 0.5M H2SO4 gradually disappeared with subsequent cycling (C1), along with the concomitant rise in the Au oxide reduction peaks (D1), indicative of gradually dissolution of Pd. Importantly, the lost Pd surface surprisingly regenerated (C2) with subsequent aging in 0.1M KOH media, particularly at sweeping potentials extending to very positive values. The regenerated Pd-rich surface is attributed to preferential diffusion of Pd from particle cores- to the surface aided by surface-reconstruction via a ‘place-exchange’ between adsorbed-oxygen and Pd. Detailed analyses of the XPS Pd-3d3/2 and Au-4f7/2 edges and the respective STEM-EDX elemental profiles are in good agreement with the changes of surface-structures proposed from the cyclic voltammetry measurements. All the alloys were simultaneously tested for oxygen reduction reaction (ORR). [1] Gasteiger, H. A., et al. (2005). Applied Catalysis B: Environmental, 56(1-2), 9-35. [2] Xu, J. B., et al. (2010). International Journal of Hydrogen Energy, 35(13), 6490-6500. [3] Sasaki, K., et al. (2010). Angewandte Chemie International Edition, 49(46), 8602-8607. [4] Rand, D. A. J., et al. (1972). Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 35(1), 209-218. [5] Reichenberger, S., et al. (2019). ChemCatChem, 11, 4489-4518. Figure 1